When I connected to the iDisk in the Finder, the image preview showed the old image I was trying to replace. Taking the brute force solution, I dragged it to the trash. I was warned that it would be deleted immediately, which was fine. But then I received the infamous "Sorry, you can't delete this file, it's busy" message. I've never seen this dialog on an iDisk, only on my local disk.
The fix turned out to be easy, if not obvious (thanks Kirk!) -- you have to unmount and remount the iDisk. The only thing that makes this a bit tricky is trying to figure out how to do that. It's not possible (as far as I know) from the Finder, but a quick trip to the Terminal will take care of the problem. Just type:
umount -f idisk_name/
Where idisk_name is your .Mac username. You can then return to the Finder, re-mount the disk by clicking the shortcut, and (hopefully) your busy file is no longer busy ... at least it worked for me!

